1. Field of the Invention
The present invention relates generally to electro-optical devices, and more specifically to electro-optical devices suitable for phase/intensity modulation and switching.
2. Description of the Related Art
Phase/intensity modulators and switches using an electro-optic material are in common use in the Telecom industry. An optical-electrical device, such as a Mach-Zehnder interferometer, can be used to bias an optical signal. These devices are typically driven by both a bias voltage, which sets the operating point, and an RF voltage, which is responsible for the actual modulation.
FIG. 1A shows a first prior-art Mach-Zehnder type electro-optical device 1. An optical waveguide 10 is embedded in the top surface 6 of a substrate 5. The substrate 5 is an x-cut LiNbO3 material. The optical waveguide 10 forks into two parallel optical pathways which later rejoin into a single optical pathway. The active regions of the electro-optical device, where the optical pathways are exposed to electromagnetic fields, are divided between a RF electrode region 20 and a bias electrode region 40. In the RF electrode region 20 two pairs of RF electrodes 21, 22, extend between and along the outside of the parallel optical pathways. The RF electrodes are made of metal and are separated from the substrate by a dielectric buffer layer 30. The RF electrodes are connected to a high-frequency power source.
In the bias electrode region 40 two pairs of bias electrodes 41, 42, extend between and along the outside of the parallel optical pathways. The bias electrodes are made of metal and are applied directly on top of the substrate (no buffer layer). The bias electrodes are connected to a DC or low-frequency power source.
FIGS. 1B and 1C show a vertical build-up of the electro-optical device 1 of FIG. 1A, sectioned along A–A′ and B–B′, respectively. The buffer layer 30 is often made of SiO2 and is placed between the substrate 5 and pairs of RF electrodes 21, 22 (FIG. 1B), to achieve velocity matching between the high-frequency electromagnetic fields and optical fields. This buffer layer is absent in the bias electrode region 40 (FIG. 1C), because the presence of a dielectric layer between bias electrodes and LiNbO3 substrate is known to be the main cause of long term (bias) drift. Thus, with this configuration, long-term drift is minimized, however, the optical Mach-Zehnder structure in the present example must be overly long to accommodate the two sets of electrodes.
Alternate prior art devices combine the bias and RF electrodes onto a common set of electrodes. To ensure electrical independence between bias and RF electrodes, an external electrical circuit (bias tee) is generally used. With this approach, the structure is shorter that the prior-art device of FIGS. 1A–C, since no additional space is required for a separate bias electrode. Unfortunately, with this design it is not possible to avoid the having the buffer layer underneath the bias electrode, which is disadvantageous in terms of long-term (bias) drift. Further, positioning the bias electrode on top of the buffer layer increases the distance between the electrodes and the optical waveguides, reducing the efficiency of the system and making it unsuitable for high-efficiency applications.
Therefore, there exists a need for an electro-optical device for phase/intensity modulation and switching with improved efficiency, operation, and reduced size.